The present invention relates to a drill comprising a shaft with chip-conveying flutes and a drill head at the front of the shaft, with two or more cutting elements. The cutting elements comprise at least one cutting edge which is delimited between a chip breaking surface and a relief surface and which, at least in the vicinity of the geometric center or rotational axis of the drill, comprises a curved section.
A drill of this type is previously known from SE-B-440 324 (and SE-A-7812393-2). This known drill has good cutting ability and enables drilling with fast feed. However, in practice it is marred by the disadvantage of an inferior centering ability. In relation with a reference plane that cuts the central rotational axis of the drill at an acute angle of 60.degree. to 70.degree. to a main plane that is parallel with the straight main sections of the cutting edges (or about centrally in the sectors being defined by the ends of the chip-conveying flutes), the curved section of each cutting edge protrudes a bit past this reference plane, at the same time as the relief surface of each cutting element, which relief surface is situated behind the cutting edge in the direction of rotation, extends as an uninterrupted surface up to the curved section of the cutting edge. Moreover, it is preferred that the curved section starts to curve in the immediate proximity of the central axis of the drill. This geometry produces two cutting edges that form a chisel-like configuration with a pronounced longitudinal extension in the area where the edges converge. When this chisel configuration is put against a work piece for drilling and entering into the same, it tends to slide transversely upon the surface of the workpiece.
In practice, the entering surface of the workpiece is rarely perfectly plane and rarely oriented absolutely perpendicularly to the rotational axis of the drill. On the contrary, the entering surface is often rough or uneven and can be inclined locally or entirely one or more degrees in relation to the drill axis. Also with a very forcible placing of the drill against the workpiece, the chisel configuration of the drill tends to move at least a little transversely, without any capacity of finding a distinct centration. This tendency is particularly pronounced just at the entering of the workpiece, but the generally bad centering ability is also present when the drill head has worked its way into the workpiece, causing vibrations in the workpiece and in the drill itself. These inconveniences also become noticeable when working with relatively long drills (drills whose length is equal to or exceeds 3.5 times the diameter of the drill), particularly in connection with crank fixations of the workpiece. When vibrations arise, these negatively influence the quality of the drilled hole and the durability of the drill.
One object of the present invention is to eliminate the above mentioned inconveniences of previously known drills and create a drill with good centering ability. Thus, a primary object of the present invention is to confer to the head of the drill such a geometry that it becomes self-centering at the placing against and the initial penetration of the drill into a workpiece. The drill also maintains a good centering ability during the continuing penetration of the workpiece, whereby the good centering ability shall be possible to achieve without the necessity of large feeding or placing forces. Another object of the present invention is to attain a good centering ability without the necessity of substantially reducing the active cutting edge length.
According to the present invention, at least the primary object is brought about by the drill as set forth in claim 1. Further preferred embodiments of the present invention are defined in the dependent claims.